Stepwise Refinement

Stepwise Refinement by Functional Decomposition

• Stepwise refinement: design a problem solution by
1. stating the solution at a high level
2. refining steps of the solution into simpler steps
3. repeating step 2, until steps are simple enough to execute
   
• Decompose based on function of each step
  
• Makes heavy use of pseudocode
  

Example

• Problem: Print the count and average of a sequence of positive integers


• Initial Solution:
  
1. Initialize data
   
2. Get data
3. Calculate results
   
4. Output results
   


• Refinement:
1. Initialize data
2. get data
• 2.1 loop
• 2.2     get integer x
• 2.3     exit when x < 1
• 2.4     process x
3. Calculate results
• 3.1 avg = sum / count
  
4. Output results
   

• Further refinement (of step 2.4):
• 2.4.1 increase count by 1
• 2.4.2 increase sum by x


• Process of refinement is complete when all steps can be easily programmed

Application to Module Design

• Same technique can be used to decompose a system into modules
  
1. Begin with main module
2. Break a module into submodules
3. Repeat step 2 until modules are small and easily written
4. Use stepwise refinement on individual modules

Module Design Goal

• According to David Parsons (and others) a good design should
• Maximize Cohesion
• Minimize Coupling


• Cohesion: Procedure have one well defined task
• Coupling: Don't access global data or data from another module


• 324 Students: Does this sound familiar?

Structure chart

1. Modules


2. Parameters and return values
1. logical
   
2. data


3. If and loop

Example

• Update customer master file with transaction records

Top Down and Bottom Up

• Note that the example above is a top down solution


• Top down: solve a problem by breaking a solution into smaller and smaller parts


• Bottom up: solve a problem by joining simpler functions to make more and more complex functions


• Most people use a combination of top down and bottom up

Top Down vs Bottom Up

• Advantages of top down
• Relatively easy
• Defer implementation details (put in bottom modules)
  
• Top down testing


• Disadvantages of top down
• Hard to identify top functions (is there a simple function at the top?)
• No natural hierarchy


• Bottom up strength
• Identifies system utilities that can be reused


• Bottom up weakness
• Hard to do entire system from bottom up